The industry has enjoyed tremendous growth for wireless systems operating in licensed and unlicensed frequency bands. With this growth, interference issues have increased especially related to adjacent channel interference (ACI) from other wireless systems. In the case of licensed band systems, ACI is mitigated because of the protection afforded by the restrictions of the licensed band. Also, in the case of indoor unlicensed band systems, ACI is mitigated by controlling the wireless devices operating within an indoor environment. However, for the case of outdoor unlicensed wireless systems, there may not be an external method to mitigate ACI from other wireless systems. Hence, a wireless system operating outdoors in an unlicensed band may create significant risk of interference because the operators may not coordinate the emissions in the frequency spectrum.
FIG. 1 illustrates in frequency spectrum 100 a typical environment for an WLAN environment with ACI. As shown, the frequency spectrum 101 of the information signal has sufficient signal power to achieve an appropriate signal to noise ratio (SNR) compared with the interference 102 that is located in the desired band. The performance of a radio is also determined by the signal-to-interference ratio (S/I or SIR) 105, which is defined as the ratio of the data signal to the interference signal. SIR 105 is often more critical to radio performance than the signal-to-noise (SNR) ratio 104. The design of wireless systems, including the wireless system's RF sub-system and digital filtering, may greatly affect the performance of the wireless system and the achievement of an acceptable SIR and SNR.
Consider frequency spectrum 100 with the presentence of strong ACI. Adjacent to the frequency spectrum 101 of the information signal is the frequency spectrum of the adjacent channel interference 103. In a controlled indoor environment, the adjacent channel interference 103 is likely “out-of-band” interference. “Out-of-band” refers to frequencies that are not within the frequency band or channel of the desired channel or signal. Hence, the out-of-band filters of the radio receiver may be sufficient to remove the out-of band ACI.
However, if the frequency spectrum of adjacent channel interference 103 is “in-band” relative to the desired channel associated with the frequency spectrum 101 of the information signal, then it may be more difficult for the radio receiver to mitigate a strong ACI signal. This describes the challenge that IEEE 802.11 systems and other radio systems need to address in an out door environment. This situation creates a need to filter interferers that are located in the desired band spectrum.
The ability of an RF system to reject interference emanating from adjacent channels is highly dependent upon the receiver architecture. Adjacent channel rejection (ACR) is a measure of how much ACI a receiver may tolerate and continue to provide acceptable performance. One of ordinary skill in the art may recognize that receiver architectures for IEEE 802.11 WLAN systems may be direct conversion or dual conversion. Further, the dual conversion architecture may be implemented as a superheterodyne (superhet) architecture. Although superhet architectures offer performance advantages, the economics of direct conversion architectures has resulted in the majority of the WLAN receiver integrated circuits (IC) to be implemented with a direct conversion architecture.
However, direct conversion receivers have limited filtering capabilities and limited dynamic range. Hence, WLAN systems designed with direct conversion architecture ICs are limited in their ability to mitigate strong desired band ACI.
One of ordinary skill in the art may recognize that with the direct conversion architecture and in a strong ACR environment, the radio may obtain an interference signal at the radio's analog to digital converter in the receiver chain that may be 40 db stronger than an acceptable level. On reason for this situation is that direct conversion architecture may not have the surface acoustic wave (SAW) filter at the intermediate frequency (IF), resulting in an interference signal at the A/D converter in the receiver chain that 40 dB stronger than the acceptable level. Accordingly, the filtering provided by a superhet receiver architecture reduces ACI to permit an acceptable performance for WLAN systems. Direct conversion architectures, however, are generally not able to provide acceptable performance with strong ACI.
Hence, while direct conversion architectures are acceptable for unlicensed indoor environments, they are generally not acceptable for outdoor environments. Given the popularity of low cost WLAN ICs with direct conversion architecture, there is a need to improve these WLAN systems based on direct conversion architectures to permit operation in an outdoor environment with high adjacent channel interferers.